The typical geometry of a conventional hyperbolic Cassegrain antenna comprises a primary feed horn, a hyperbolic subreflector, and a paraboloidal main reflector. The central portion of the hyperbolic subreflector is shaped and positioned so that its virtual focal point is coincident with the phase center of the feed horn and its real focal point is coincident with the virtual focal point of the parabolic main reflector. In the transmitting mode, the feed horn illuminates the subreflector, the subreflector reflects this energy in a spherical wave about its real focal point to illuminate the main reflector, and the main reflector converts the spherical wave to a planar wave across the aperture of the main reflector. To suppress wide angle radiation, the antenna employs a cylindrical absorber-lined shield on the main reflector. In the receiving mode, the parabolic main reflector is illuminated by an incoming planar wave and reflects this energy in a spherical wave to illuminate the subreflector, and the subreflector reflects the incoming energy into the feed hom.
The geometry of a typical prime-fed antenna comprises a feed horn with a button-hook, and a parabolic main reflector. The central portion of the parabolic main reflector is shaped and positioned so that its virtual focal point is coincident with the phase center of the feed horn. In the transmitting mode, the feed horn illuminates the main reflector, and the main reflector radiates a planar wave across the aperture of the main reflector. To suppress wide angle radiation, the antenna employs a cylindrical absorber-lined shield on the main reflector. In the receiving mode, the parabolic main reflector is illuminated by an incoming planar wave and reflects the incoming energy into the feed horn.
Usually, the above antennas must radiate substantially symmetrical patterns with equal E-plane and H-plane radiation patterns. The E-plane pattern corresponds to horizontal polarization and the H-plane pattern corresponds to vertical polarization. To radiate symmetrical patterns from either the hyperbolic Cassegrain antennas or the prime-fed antenna, the feed horn must radiate approximately equal E-plane and H-plane patterns. A corrugated horn radiates approximately symmetrical radiation patterns; however, a corrugated horn is not a preferred design choice because of its high construction cost, especially at millimeter wavelength frequencies corresponding to 20 to 60 gigahertz ("GHz" hereafter) range. Instead of implementing the costly corrugated horn, a dual mode ("DM" hereafter) horn may be used. The DM horn radiates TE.sub.11 and TM.sub.11 modes and has a low construction cost.